Comments on: Wigner Crystals https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/
Fri, 24 Aug 2018 04:08:47 +0000
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By: John Baez https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-127455
Fri, 24 Aug 2018 04:08:47 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-127455Yes, that’s what I said in my article.
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By: Pradeep Kumar https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-127219
Thu, 23 Aug 2018 15:49:59 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-127219An electron gas where the particle motion is limited in two dimensions but the field lines are not limited, will have 1/r interaction.
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By: John Baez https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101388
Mon, 11 Dec 2017 16:36:59 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101388There are very nice results in this paper – for example, a description of some common defect patterns. Such patterns should appear in a large 2d Wigner crystal for a large variety of potentials (i.e. parabolic potential, square well potential on a disc, etc.).
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By: John Baez https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101372
Sun, 10 Dec 2017 17:06:01 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101372Nice! Maybe you should add some information about particles on discs to your list. This is the best thing I’ve found so far, but it has references to more:

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By: HaroldW https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101348
Sat, 09 Dec 2017 14:44:00 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101348An interesting contrast is “Topological defect motifs in two-dimensional Coulomb clusters” by Arunas Radzvilavičius and Egidijus Anisimovas where the particles are confined by a parabolic energy well, rather than a square well. They write, “Distribution of defects in a parabolic confinement is by large determined by a conflict between the circular boundary and the bulk-like interior where a hexagonal lattice is preferred. Small clusters (N ≤ 70) have no bulk and exhibit a shell structure defined by the circular symmetry of the confinement.”

Their Figure 5 (N=520) is similar to the figure at the top of your post.

One interesting thing I hadn’t guessed is that the particles often lie on concentric shells.

Abstract. The lowest energy configurations for N equal charged particles confined to a thin conducting disc have been investigated in detail up to N = 160 and in outline for further values up to N = 500. For all values of N up to 160 the particle configurations can be described in terms of concentric shells. The number of perimeter particles p appears to be simply related to N and to the mean radius of the outermost internal shell. Justification for these relations is obtained from a simple model based on the well-known distribution of continuous charge on a conducting disc.

In the N → ∞ limit the density of particles in the unit disk is proportional to

Other interesting things:

In the range N ≤ 80, the configuration for N + 1 is, with one exception, the same as that for N with the addition of one particle to an existing shell or the creation of a new shell with a particle placed at the centre. The single exception is the pair N = 55 (with shell structure 5-13-37) and N = 56 (with shell structure 1-6-12-37). For the range 81 ≤ N ≤ 160 there are four exceptions to the usual pattern: N = 97–98, 117–118, 150–151 and 152–153.

The first case where the particles don’t lie on concentric shells is N = 185.

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By: John Baez https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101319
Fri, 08 Dec 2017 17:21:20 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101319The main reason all the charge doesn’t go to the boundary of the disk is that we’re working in two dimensions yet using the potential that’s the solution of the Poisson equation

in three dimensions. If we minimized potential energy for the potential that solved Poisson’s equation in two dimensions, the charge particles would indeed cluster on the boundary of the disk…

… at least in the limit where the number of particles approaches infinity.

In the 3-dimensional case, for finite numbers of particles confined to a ball minimizing the potential energy

the particles don’t always lie on the surface of the ball! In the 2-dimensional case, with the potential energy appropriate to 2 dimensions

I think the case of 7 particles would be a good one to try, to see this effect.

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By: John Baez https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101318
Fri, 08 Dec 2017 16:57:49 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101318Hre are pictures of configurations of points on the 2-sphere that minimize the energy

The pictures go up to 75 points, with some gaps. Here is the case of 72 points, where apparently there are some squares:

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By: A https://johncarlosbaez.wordpress.com/2017/12/07/wigner-crystals/#comment-101314
Fri, 08 Dec 2017 14:27:16 +0000http://johncarlosbaez.wordpress.com/?p=24092#comment-101314I’d imagine you want to look into Burgers vectors if you haven’t already, since there are conservation laws related to them that can be applied to defects in crystals. Decent summary can be found in this book: